Electrolysis of zinc chloride



MalCh 23, 1954 D. w. DUNCAN ET AL ELEcTRoLYsrs oF ZINC CHLORIDE Filed Sept. l, 1950 IN VEN TOR .Dama Mr/4mm Perea [wr/.f

Patented Mar. 23, 1954 2,673,178 ELEornoLYsIs F ZINC oHLoRIDE Daniel W. Duncan, Portsmouth,

Norfolk County,

and Peter Eustis, Va.

Application September 1, 1950, Serial No. 182,651

5 Claims. l

This invention relates manufacture of zinc dust to the electrolysis of zinc chloride solutions to produce zinc dust and chlorine gas.

Theoretically, the electrolysis of zinc chloride solutions possesses many advantages over the electrolysis of zinc sulfate solutions for the production of metallic zinc. One of the advantages is the possibility of obtaining chlorine gas as one of the products of the electrolysis. Other advantages are that solutions of higher zinc 'concentration may be electrolyzed and the voltage required for the electrolysis of zinc chloride is lower than that for zinc sulfate. However, serious problems are encountered in the electrolysis of zinc chloride. One of the important problems is the re-solution of the zinc and corrosion at the cathode by solutions containing chlorine gas. Another important difficulty arises from the sensitivity of the electrolysis of zine chloride to the presence of impurities and the dii'culty in maintaining control of the electrolysis to obtain a satisfactory product. The net result is that the disadvantages in the electrolysis of zinc chloride, as compared with zinc sulfate, have outweighed the advantages and all of the electrolytic zinc plants in existence plate from sulfate solutions.

In galvanizing processes important ley-products containing large quantities of zinc are sal skimmings and galvanizers skimmings. Both of these materials constitute zinc. However, both are contaminated with chlorides. No satisfactory process for the electrolysis of mixtures of zinc sulfate and chloride has been developed, in part because of the corrosion problem inasmuch as materials capable of withstanding attack by both chlorides and sul.- fate and suitable for use in electrolytic cells are not commercially available. For this reason when chloride contaminated zinc-bearing raw materials have been used, it has been necessary to remove the chlorides, for example by roasting processes, prior to electrolysis, and the subsequent electrolysis is of sulfatesolutions.

This invention has as an object a process for the electrodeposition of zinc from zinc chloride solutions.

A further object of this invention is the provision of a process for the preparation of an improved metallic zinc dust.

Another object of this invention is to provide a process for the electrolysis of zinc ehloridesoluetions to obtain concentrated chlorine gas and metallic zinc dust.

to a process `for the i It 1s also an object of this invention to4 pro`` and more particularly relatively low cost sources of the process of this grams of zinc per liter.

vide a novel electrolytic cell allowing the con-V tinual and simultaneous removal of chlorine and zine dust therefrom.

With these and other objects in view, which will become apparent in the following detailed description of the process of this invention, this invention resides in the electrolysis of zinc chloride solutions for the production of metallic zinc dust and the simultaneous recovery of gaseous chlorine.

In the drawings:

Figure 1 is a vertical longitudinal sectional view of an embodiment of anl electrolytic cell for invention.

Figure 2 is also a vertical sectional view. taken along section line 2--2 in Figure 1.

The electrolysis of zinc chloride according to this invention is performed in an electrolytic cell having insoluble anodes and diaphragms separating the cell into anode compartments and cathode compartments. These diaphragms allow the removal of anolyte and aid in the collection of the chlorine gas liberated at the anode of the cell. The structure of the cell is described hereinafter.

The electrolyte in the cell is a relatively pure solution of zinc chloride which may be obtained from any suitable source. In view of the liberation of concentrated chlorine gas at the anode of the electrolytic cell, which gas may be dissolved in water and employed as an eicient leaching agent for sal skimmings from hot dip galvanizing processes, sal skimmings provide a desirable source of zinc chloride in the form of a strong zinc chloride solution. Sal skimmings contain zinc in the form of metallic zinc, zinc oxide and zinc chloride, the chloride content varying between about 2 and 35%. Iron, manganese, and other impurities in the sal skimmings are oxidized by the chloride, precipitated, and removed from the electrolyte by iiltration prior to use in the cell.

The concentration of zinc chloride in the catholyte may vary broadlyv from about 15-80 The lower limit of the zinc concentration is rather sharply defined. If the zinc concentration drops below 18 grams per liter operation of the cell to provide zinc dust is diiiicult and if it drops below I5 grams per liter satisfactory operation of the cell cannot be'maintained. At a concentration of 2'0 grams per liter and above, smooth operation of the cell may be maintained without difculties resulting from the low concentration of zinc in the catholyte, and the preferred concentration` of zinc in the* catho# 3 lyte is about 2li-l0 grams per liter. Concentrations of zinc above 80 grams per liter cause excessive deposition of zinc plate. Ihe desired concentration of zinc in the catholyte is maintained by the addition of a strong solution of zinc chloride to the catholyte, and withdrawal of anolyte from the cell in the manner hereinafter described.

The catholyte is maintained slightly acidic for the deposition of zincV dust and must be maintained within the limits of a pH of 4.4-5.8 in the cathode compartment. It is preferred to operate within the narrow limits of a pH of 4.8-5.3 for most eiicient operation.V If the catholyte is permitted to become more acidic, zinc tends to plate out at the cathode, whereas lower acidities will result in the formation of zinc hydroxide. Usually, it is desirable to add sodium chloride in concentrations as high as 300 grams per liter to the electrolyte to improve its conductivity.

The electrolytic cell is operated at a low current density between the limits of and 75 amperes per square foot and preferably between 25 and 60 amperes per square foot. lfrhigher current densities are employed, the efeiency of operation drops sharply both as a result of the higher voltage necessary to maintain the higher current densities causingV a lower power efficiency and the introduction of side reactions utilizing the current through the cell to deposit products other than metallic zinc, principally oxides and hydroxide. These side reactions encountered at high current density thus are harmful in decreasing the purity of the zinc dust as well as causing lower current efficiencies.

An indication of the importance of the current density from the standpointof the purity of the product is that the operation of an electrolytic cell under optimum'conditions and a current dem sity within the range of 25-60 amperes per square foot will result in about 90% of the zinc being deposited in the form of metallic zinc dust. On the'other hand, if the current density is increased to about 90 amperes per square ioot, and the other variables are adjusted to maintain the maximum efciency of operation possible at that current density, only about '10% of the Zinc will be deposited as metallic Zinc dust.

The electrolytic cell may be operated within wide temperature limits which may vary from as low as C. to as high as 90 C. n general, it is preferred to operate within the limits of -60 C. While the temperature may vary widely it has an important effect on the adjustment of the other variables for operating the cell at maximum eiiiciency. For example, if the temperature has been increased, it will be necessary to lower the zinc concentration in the electrolyte to give the maximum overall efficiency. rThe limit on reduction of the concentration of zinc in the electrolyte to counteract increases in temperature will be reached when zinc hydroxide is precipitated at the cathode. Similarly, if the current density is reduced it will be necessary to reduce the temperature of the cell to maintain the maximum emciency of operation. Hence, the operating variables of the cell are interdependent and a proper balance of all of the variables within the limits set forth is necessary for the operation of the cell at maximum efciency.

- It has been found desirable for the operation of the process of this invention at acceptable eiciencies to circulate the electrolyte through the cell at a high rate of flow.V The circulation may be as much as 60 times the rate o circulation employed in those` processes for the production of zinc plate which employ circulation of the anolyte and is accomplished by withdrawing anolyte from the anode compartment and returning it to the cathode compartment. The anolyte will be slightly more acidic than the catholyte and will be slightly depleted in Zinc content, hence its composition is corrected by the addition of zinc chloride solution of the proper strength and pH before returning to the cathode compartment. This high rate of circulation, combined with the low current densities employed in this process, result in a very low depletion of zinc from the electrolyte in each pass through the cell. The Zinc depletion should not exceed 20 grams per liter of the electrolyte in each pass and a depletion as low as one gram of zinc per liter of electrolyte per pass is often employed.

The reason for the necessity of circulating the electrolyte is not known, but if these relatively high rate of iiow are not maintained a low efficiency of operation of the cell is obtained. Inasmuch as the high rates of circulation are required even when the zinc concentration in the electrolyte is high, depletion of the zinc concentration of the electrolyte apparently is not the l sole reason for requiring high rates of circulation.

Y ing conditions. f

The high rates of circulation may also be of Value in preventing the diffusion of chlorine gas from the anode compartment to the cathode and redissolving the zinc dust, and other side reactions.

For efficient production of zinc dust, it has been found necessary to employ an electrolytic cell having a porous diaphragm through which the electrolyte may pass separating the cell into an anode compartment and a cathode compartment. The characteristics, such as thickness, of the diaphragm influence the desired flow rate. It has been found that the thickness of the diaphragm and the rate of flow from the cathode 'to the anode should be such that a pressure drop of at least 1.0 millimeter of Avvater across the diaphragm should be maintained to obtain good operation. In most instances, it will be desirable to maintain a pressure drop oi at least 5 vmillimeters of water across the diaphragm, and

the preferred rate of circulation is such that a pressure drop of about l2 millimeters of water across the diaphragm is maintained.

' The iniiuence of the characteristics of the diaphragm on the rate of circulation of electrolyte is illustrated in the following examples.

EXAMPLE I A diaphragm consisting of a single thickness of a woven fabric conventionally used as a lter cloth was supported between the anode and cathode of a cell. At a pressure drop of 12 millimeters of water, the flow rate of the electrolyte was cubic centimeters per square foot per minute.

EXAMPLE II A diaphragm was prepared from two thicknesses of the fabric employed in Example I and three thicknesses of heavy, short iiber asbestos paper. A flow of only d0 cubic centimeters per square foot per minute produced a pressure drop across the diaphragm of 12 millimeters of water.

Electrolytic cells having the diaphragms and ow rates set forth in Examples I and Il have comparable efficiencies for a given set of operat- It will be appreciated that the depletion of the catholyte will be greater at the low rates of circulation employed with thick diaphragms of increased resistance to flow when acted-"r the same current densities are maintained. For example, it is practical to operate with high eiliciency with feed solutions having concentrations as high as 35-40 grams of zinc per liter by operating with a diaphragm having relatively* high resistance to iiow and maintaining a high zinc depletion, for example, 15-20 grams per liter.

The high rates of circulation of the electrolyte generally required to maintain the desired pressure drop and the resulting low depletion of zinc concentration, result in the cell being operated with a small difference in the composition of the anolyte and the catholyte in the cell. This is of value in increasing the stability of operation of the cell inasmuch as the electrolyte introduced into the cathode compartment will have a composition very close to that of the electrolyte already present. In View of the necessity for the higher rates of circulation, the electrolysis of zinc chloride to produce gaseous chloride at the anode is highly advantageous because the chlorine gas serves as a strong acid suitable for leaching raw materials such as sal skimmings. If a zinc sulfate solution, for example, is electrolyzed and the high rates of circulation employed in this invention are maintained, the liquid withdrawn from the anode compartment will be a very weak solution of sulfuric acid having little value as a leaching agent.

Chlorine is liberated at the anode of the cell as a valuable by-product of the electrolysis and is collected and carried away from the cell for liquefaction or other use. The zinc is deposited at the cathode in the form of a dust particularly suitable for use in chemical reduction processes. Some of the zinc dust will adhere to the cathode, hence, the cathodes are periodically removed from the cell and scraped to recover the zinc dust without further treatment.

Referring to the drawings, a preferred embodiment of an electrolytic cell to be used in this invention is illustrated having a casing I in which a plurality of alternating cathodes 2 and anodes 3 are suspended. While the cell has been shown as having a plurality of cathodes and anodes, clearly the concept of this invention includes a cell having a single anode and cathode. The anodes may be supported in any suitable manner, such as by hangers tl which extend from rails 5 extending along the side of the casing I. The cathodes 2 are similarly supported by hangers 6 resting on rails 1 and extending inwardly over the electrolytic cell for attachment to the cathodes. In the form of the invention shown, the anodes 3 are connected to a bus bar 8 at the side opposite the hangers 4, whereby the bus bars aid The bus bars 8 and 9 are electrically connected to suitable conductors I I and I2, respectively, which extend along the side of the cell for the delivery of current to the electrodes.

A hood I3 surrounds the upper portion of each of the anodes for the collection of chlorine liberated at the anodes. Each hood I3 extends downwardly along the anode below the level of the take-olf line I4 for collection of chlorinev gas liberated at the anode and delivery of the gas to a suitable header I5. The hood I3 is insulated and sealed from the bus bar 8 at I6 where the bus bar extends through the hood. Clearly, the anode may extend upward above the hood I3 and 6. be connected to the bus bars above the hood. A structure of this type is desirable in that it keeps the chlorine away from the bus bars. I

The lower portions of the anodes 3 are enclosed by diaphragme I'I which divide the electrolytic cell into anode compartments and cathode compartments. The diaphragms are conventional diaphragms employed in electrolysis operations and are porous to permit flow of electrolyte to the anode and also permit flow of current through the cell. The diaphragms I'I extend upwardly into the lower end of the hood I3 which is secured thereto in any suitable manner such as strips extending around the upper end of the diaphragm and bolted to the lower end of the hood I3 in the manner illustrated -in Figure 2.

Diaphragme I1 are provided with openings I8, preferably in their lower end, for the reception of drain lines I9. The drain lines I9 are connected to a header 2l for the removal of anolyte from the anode compartments of the cell to permit recirculation of the anolyte through the cell. Header 2I extends through the sides of the casing and is provided with a suitable swing joint 22 and a stand pipe 23 opening at its upper end 24. Swing joint 22 permits the stand pipe 23 to be adjusted to any desired level and thereby adjusts the level of the electrolyte within the anode compartments for control of the rate of flow of the electrolyte through the diaphragms I'I. The upper end 24 of the stand pipe 23 opens over a storage vessel 25 for the anolyte.

The suction line 25 of a circulating pump 2'I extends into the storage vessel, the pump 21 having a discharge line for delivering the anolyte into a circulating header 28. Individual delivery lines 29 take on from the header 28 for delivery of the recirculated anolyte into the cell adjacent each of the cathodes. A strong liquor supply line 3I is also connected with the header 28 for the replenishing of the zinc concentration in the The bottom of casing l has steeply sloping walls 32 as illustrated in Figure 1 for the collection of zinc dust as it drops from the cathode. The steeply sloping walls 32 direct the zinc dust through an opening 33 allowing removal of the solid material while the cell is in operation. In

may be employed, for example, it may be preferred to provide a conical bottom to the casing I below the cathodes for the removal of the zinc dust.

The novel electrolytic cell of this invention allows simultaneous collection and removal of zinc dust and chlorine gas. Moreover, some of the cathodes ofthe cell may be disconnected from the bus bars and lifted directly out of the cell for sera-ping without interrupting the cell operation. The provision of the drain lines from the anode compartments and the recirculating mechanism allows accurate control of the characteristics of the electrolyte and provides the recirculation of anolyte required for the eiicient production of zinc dust,

7 Y While this invention has 'been described with' reference to aY specc embodiment, its scope is not limited to the details described but is defined by the appended claims.

We claim:

1. A process for the production of metallic zinc dust comprising electrolyzing an electrolyte comprising a solution of zinc chloride containing about 15-80 grams of zinc per liter in the catholyte and a pH of 4.4-5.8 in an electrolytic cell maintained at a temperature of 20-90 C. having insoluble anodes at a current density of 10-'75 amperes per square foot to deposit zinc dust at the cathode and release chlorine gas at the anode, withdrawing electrolyte from adjacent the anodeV of the cell and returning the electrolyte to the cell adjacent the cathode, and adding a strong solution of zinc chloride to the electrolyte to replenish the zinc deposited therefrom.

2. A process for the production of metallic zinc dust comprising electrolyzing an electrolyte comprising a solution of zinc chloride containing about 20-40 grams of zinc per liter in the catholyte and a pH of 4.8-5.3 in an electrolytic cell maintained at a temperature of 20-60 C. having insoluble anodes at a current density of 25-60 amperes per square foot to deposit zinc dust at the cathode and release chlorine gas at the anode, withdrawing electrolyte from adjacent the anode of the cell and returning the electrolyte to the cell adjacent the cathode, and adding a strong solution of zinc chloride to the electrolyte to replenish the zinc deposited therefrom.

3. A process for the production of zinc dust and chlorine comprising electrolyzing a solution of zinc chloride containing 15-80 grams of zinc per liter in the catholyte and having a pH of 4.4-5.8 at a current density of 10-75 amperes per square foot in a cell having insoluble anodes and a diaphragm separating the anodes from the cathodes, and maintaining said cell at a temperature of 20-90 C.

4. A process for the productionV of zinc dust and chlorine comprising electrolyzing a solution of zinc chloride containing 20-40 grams of zinc per liter in the catholyte and having a pH of 4.8-5.3 at a current density of 25-60 amperes per square foot in a cell having insoluble anodes and a diaphragm separating the anodes from the cathodes, and maintaining said cell at a temperature of 20-60 C. f

5. A process for the production of zinc dust and chlorine comprising electrolyzing a solution of zinc chloride containing 15-80 grams of zinc per liter in the catholyte and having a pH of 4.4-5.8 at a current density of 10-75 amperes per squarefoot in a cell having insoluble anodes and a diaphragm separating the anodes from the cathodes, and maintaining said cell at a temperature ofV 20-90 C., withdrawing anolyte from the cell at a rate sufcient to maintain a. pressure drop greater than one millimeter of water across the diaphragm and returning it to the vicinity of the cathode, and maintaining the pH and the concentration of zinc in the electrolyte by adding zinc chloride solution to the recirculating anolyte. Y

DANIEL W. DUNCAN. PETER EUSTIS.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 405,604 Revello June 18, 1889 733,028 Goldberg July '7, 1903 961,514 Mojana June 14, 1910 1,732,797 Eustis Oct. 22, 1929 2,313,338 Hannay et al. Mar. 9, 1943 2,421,265 Hogaboom May 2'7, 1947 2,446,983 Prust Aug. 10, 1948 FOREIGN PATENTS Number Country Date 17,205 Great Britain of 1889 '720,663 Germany May 12, 1942 

1. A PROCESS FOR THE PRODUCTION OF METALLIC ZINC DUST COMPRISING ELECTROLYZING AN ELECTROLYTE COMPRISING A SOLUTION OF ZINC CHLORIDE CONTAINING ABOUT 15-80 GRAMS OF ZINC PER LITER IN THE CATHOLYTE AND A PH OF 4,4-5.8 IN AN ELECTROLYTIC CELL MAINTAINED AT A TEMPERATURE OF 20-90* C. HAVING INSOLUBLE ANODES AT A CURRENT DENSITY OF 10-75 AMPERES PER SQUARE FOOT TO DEPOSIT ZINC DUST AT THE CATHODE AND RELEASE CHLORINE GAS AT THE ANODE, WITHDRAWING ELECTROYLTE FROM ADJACENT THE ANODE OF THE CELL AND RETURNING THE ELECTROLYTE TO THE CELL ADJACENT THE CATHODE, AND ADDING A STRONG SOLUTION OF ZINC CHLORIDE TO THE ELECTROLYTE TO REPLENISH THE ZINC DEPOSITED THEREFROM. 